In recent years, researches and developments such as basic science, material development, medical application, and the like have been actively carried out for new industrial development using a large laser. To obtain a large-output laser beam, a laser beam amplification device for amplifying input seed light is required. The laser beam amplification device includes a laser medium unit and an excitation light source for causing excitation light to enter the laser medium unit. The laser beam amplification device cools a laser medium by flowing a cooling medium to contact with a main surface of the laser medium in the laser medium unit (refer to Patent Literature 1).